Method and apparatus for displaying different shades of gray on a liquid crystal display

• US 5,068,649 A
• Filed: 10/14/1988
• Issued: 11/26/1991
• Est. Priority Date: 10/14/1988
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Compaq Computer Corporation (HP Inc.)
• Inventors: Garrett, James H.
• Primary Examiner(s): Oberley, Alvin E.
• Application Number:
  US07/258,269
• Time in Patent Office: 1,138 Days
• Field of Search: 340/793, 340/784, 340/703, 358/283
• US Class Current: 345/89
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 3/2025 the sub-frames having all the same time duration
• CPC Class Codes: G09G 3/2051 with use of a spatial dither pattern
• CPC Class Codes: G09G 3/3611 Control of matrices with row and column drivers
• CPC Class Codes: H04N 3/127 using liquid crystals

Abstract

A method and apparatus are disclosed which provide a means for both spatially and temporally resolving the on/off states of a two-state display device such as a liquid cystal display to provide apparent shades of gray. A particular feature of this method is that the cycling between on and off states is not performed in a discernible pattern. Rather, a pseudo-random pattern is utilized which repeats only after many cycles. Additionally, when the method disclosed herein is utilized, adjacent pixels, when selected to display the same shade of gray, do not cycle on and off in synchronization, but rather utilize out-of-phase cycling patterns. This spatial resolution reduces perceived flicker in the display and provides a more stable image. In one embodiment, eight shades of gray are provided. The shades are generated by cycling individual pixels such that when averaged over time, they are: always off; on 20% of the time; 33% of the time; 40% of the time; 60% of the time; 67% of the time; 80% of the time; or, on at all times.

Claims

1. A display control system for producing an optical gray-scale image on a display device having an array of display elements each providing a first or a second optical state in response to a first or a second signal level, respectively, th array of display elements having a plurality of rows and a plurality of columns, the system comprising:

means for generating respective display signals for the display elements for producing a gray-scale image of a specified color, the display signals comprising digital signals each having a pattern of bits respectively corresponding to the first or the second signal level, a predefined pattern cycle and a duty cycle related to the optical gray-scale of the image at the position of the respective display element, the pattern of bits of each one of the digital signals being repetitively generated, the means for generating display signals providing successive bits of the display signals for respective diaplay elements in successive timeframes, in each timeframe one bit of each of the display signals being provided in sequence for consecutive display elements in each row from a first to a last display element of the row and for consecutive rows beginning at a first row and ending at a last row of the array, and causing a predetermined skewing of each subsequently generated display signal having a pattern cycle for which the total number of display elements in a row is integrally divisible each time a bit of a respective display signal is provided for the last display element of a row, and causing a predetermined skewing of each subsequently generated display signal having a pattern cycle for which the total number of display elements in the array is integrally divisible each time a bit f a respective display signal is provided for the last display element of the last row of the array.

2. The display control system of claim 1, wherein the display device is a liquid crystal display (LCD) panel and the array of display elements comprises an array of screen dots of the LCD panel.

3. The display control system of claim 1, wherein the means for generating the display signals for the display elements includes:

means for concurrently generating a plurality of serial digital signals each having a pattern of bits respectively corresponding to the first or the second signal levels, a predefined pattern cycle and a different duty cycle;

means for generating the first and the second signal levels;

display control means for providing address data for sequentially addressing consecutive display elements in consecutive rows of the array in each timeframe from the first display element of the first row to the last display element of the last row, color attribute data associated with each display element being addressed, an end-of-row signal when address data for addressing the last display element of a row is provided, and an end-of-frame signal when address data for addressing the last display element of the last row is provided; and

display signal selection means responsive to the address data and the color attribute data for selecting the first or the second signal level, or a respective bit of one of the plurality of serial digital signals having a duty cycle related to the optical gray-scale of the image at the position of the display element being addressed, and wherein the means for generating the plurality of serial digital signals is responsive to the end-of-row signal for skewing by a predetermined number of bit positions each subsequently generated digital signal having a pattern cycle for which the number of display elements of a row is integrally divisible, and is further responsive to the end-of-frame signal for skewing by a predetermined number of bit positions each subsequently generated digital signal having a pattern cycle for which the total number of display elements of the array is integrally divisible.

4. The display control system of claim 3, wherein the means for generating a plurality of serial digital signals includes respective feedback shift register means for generating each pair of serial digital signals having the same pattern cycle and complementary patterns of bits.

5. The display control system of claim 1, wherein the optical gray-scale image produced on the display device is composed of an array of pixels each consisting of a respective one of the display elements, the image having eight gray-scale levels and each one of the display signals having a pattern cycle of 3 or 5 and a duty cycle of 1/3 or 2/3 or 1/5, 2/5, 3/5 or 4/5, respectively, or a duty cycle of 0 or 1, where the duty cycle of 0 corresponds to the first optical state and the duty cycle of 1 corresponds to the second optical state.

6. The display control system of claim 1, wherein the optical gray-scale image produced on the display device is composed of an array of pixels each consisting of a separate group of display elements in consecutive rows and consecutive columns, and a respective optical gray-scale level is obtained for each one of the pixels by cross-hatching of the display elements of the pixel by providing respective display signals therefor.

7. The display control system of claim 6, wherein each pixel of the optical gray-scale image produced on the display device consists of a respective group of four mutually adjacent display elements, including two pairs of diagonally adjacent display elements, the image having sixteen gray-scale levels, each one of the display signals having a pattern cycle of 3 or 5 and a duty cycle of 1/3 or 2/3, or 1/5, 2/5, 3/5 or 4/5, respectively, or a duty cycle of 0 or 1, the display control system generating a respective pair of display signals having the same duty cycle for each diagonally adjacent pair of display elements of a pixel, and the duty cycles of the respective pairs of display signals for the two diagonally adjacent pairs of display elements of each pixel being 0-0, 0-1/5, 1/5.1/5, 1/5-1/3, 166 -166, 166-2/5, 2/5-2/5, 1/3-170, 2/5-2/3, 3/5-3/5, 3/5-2/3, 2/3-2/3, 2/3-4/5, 4/5-4/5, 1-4/5 or 1-1, where the duty cycle of 0 corresponds to the first optical state and the duty cycle of 1 corresponds to the second optical state.

8. A method for driving a display device having a multiplicity of display elements each providing a first or a second optical state in response to a first or a second signal level, respectively, to produce an optical gray-scale image, the display elements being disposed in an array having a plurality of rows and a plurality of columns, the method comprising the steps of:

generating respective display signals for the display elements for producing a gray-scale image of a specified color, the display signals comprising digital signals each having a pattern of bits respectively corresponding to the first or the second signal level and having a predefined pattern cycle and a duty cycle related to the optical gray-scale of the image at the position of the respective display element, the pattern of this of each one of the digital signals being repetitively generated;

providing successive bits of the display signals for the display elements in successive timeframes, in each timeframe on bit of each of the display signals being provided in sequence for consecutive display elements in each row from a first to a last display element of the row, and for consecutive rows beginning at a first row and ending at a last row of the array;

causing a predetermined skewing of each subsequently generated display signal having a pattern cycle for which the total number of display elements in a row is integrally divisible each time a bit of a respective display signal is provided for the last display element of a row; and

causing a predetermined skewing of each subsequently generated display signal having a pattern cycle for which the total number of display elements in the array is integrally divisible each time a bit of a respective display signal is provided for the last display element of the last row of the array, whereby in successive timeframes adjacent display elements in each row of the array are provided with different sequences of the first and the second signal levels, and adjacent display elements in each column of the array are provided with different sequences of the first and the second signal levels.

9. The method for driving a display device of claim 8, wherein the display device is a liquid crystal display (LCD) panel and the multiplicity of display elements is a multiplicity of screen dots of the LCD panel.

10. The method for driving a display device according to claim 8, wherein the step of generating the first and the second signal levels, and concurrently generating a plurality of serial digital signals each having a pattern of bits respectively corresponding to the first or the second signal levels, a predefined pattern cycle, nd a different duty cycle;

and the step f providing successive bit of the display signals for the display elements in successive timeframes includes the steps of;

generating address data for sequentially addressing consecutive display elements in consecutive rows of the array in each timeframe from the first display element of the first row to the last display element of the last row, generating color attribute data associated with each display element being addressed, generating an end-of-row signal when address data for addressing the last display element of a row is generated, generating an end-of-frame signal when address data for addressing the last display element of the last row is generated, and selecting the first or the second signal level, or a respective bit of one of the plurality of serial digital signals having a duty cycle representative of the optical gray-scale of the image at the position of the display element being addressed; and

wherein each subsequently generated digital signal having a pattern cycle for which the number of display elements of a row is integrally divisible is skewed by a predetermined number of bit positions each time the end-of-row signal is generated, and each subsequently generated digital signal having a pattern cycle for which the total number of display elements of the array is integrally divisible is skewed by a predetermined number of bit positions each time the end-of-frame signal is generated.

12. The method for driving a display device according to claim 8, wherein the topical gray-scale image produced on the display device is composed of an array of pixels each consisting of a separate group of display elements in consecutive rows and consecutive columns, and a respective gray-scale level is obtained for each one of the pixels by cross-hatching of the display elements of the pixel by providing respective display signals therefor.

13. The method for driving a display device according to claim 12, wherein each pixel of the optical gray-scale image produced on the display device consists of a respective group of four mutually adjacent display elements, including two pairs of diagonally adjacent display elements, the image having sixteen gray-scale levels, each one of the display signals having a pattern cycle of 3 or 5 and a duty cycle of 1/3 or 2/3, or 1/5, 2/5, 3/5 or 4/5. respectively, or a duty cycle or 0 or 1, the display control;

system generating a respective pair of display signals having the same duty cycle for each diagonally adjacent pair of display elements of a pixel, and the duty cycles of the respective pairs of display signals for the two diagonally adjacent pairs of display elements of each pixel being 0-0, 0-1/5, 1/5-1/5, 1/5-1/3, 1/3-1/3, 1/3-2/5, 1/3-2/3, 2/5-2/3, 3/5-3/5, 3/5-2/3, 2/3-2/3, 2/3-4/5, 4/5-4/5, 1-4/5 or 1-1, where the duty cycle of 0 corresponds to the first optical state and the duty cycle of 1 corresponds to the second optical state.

11. The method for driving a display device according to claim , wherein the optical gray-scale image produced on the display device is composed of an array of pixels each consisting of a respective one of the display elements, the image having eight gray-scale levels and each one of the display signals having a pattern cycle of 3 r 5 and a duty cycle of 5/8 or 2/3, or 1/5, 2/5 or 4/5, respectively, or a duty cycle of 0 or 1, where the duty cycle of 0 corresponds to the first optical state and the duty cycle of 1 corresponds to the second optical state.